Grant Support: In part by grants PM98-0064 (Spanish Ministry of Science and Technology) and NIH R01AI29193 (National Institutes of Health). Dr. Martinez-Picado was supported by contract FIS 99/3132 from the Fundacio per a la Recerca Biomedica Germans Trias i Pujol in collaboration with the Spanish Department of Health. Roche, Bristol-Myers Squibb, and GlaxoSmithKline provided industry support in the form of unrestricted grants.